how would you finger this riff?

You could either fret the two last notes simultaneously with the pinky, or even better/easier, fret the A with the pinky and the E with the ring finger at the same time with one motion.
So you can pull the plucking finger that plucks the A over the G and D string in one motion (this is also known as raking), thus playing the last two notes with one pluck motion.

 

That's how I'd do it, but using the ring finger.

 

I use double bass fingering for anything below the 12th fret which means I use the index finger, middle finger, ring and pinky to finger a fret so I use a three fret range. I would play the above riff index, middle, ring+pinky, ring+pinky. I'm already used to playing these type of figures and so have the dexterity to play them. Another tricky pattern is the following:

G(3-e) D(5-a) G(5-d) D(5-a) G(5-d) D(5-a) G(5-d) D(5-a)

Note(fret-string)

I play the above using the index finger for the G and ring+pinky for the D and the G an octave higher.

 

This is how I would do it.

 

Heh-heh-heh...
Good one, Mr. Pin Head
(...been dyin' to say that!)

I say it depends what the note is just prior to the "G"...if it's just a 1-bar figure repeated ad nauseum, I'd probably use this LH fingering: 2-3-4-4
(with the index finger stationary/fixed on the 4th fret..."F#").

Using TWO fingers to sound those two notes...IMO, that's "OK" IF...IF you desire a quasi-smooth transitional sound. Sonically, there is a difference; try both(IMO).

 

I used left hand fingers 1-2-3-2... but I'm probably wrong.